Pathways for Pyruvate
• The pyruvate produced from glucose during glycolysis can
be further metabolized in three possible ways
• For aerobic organisms, when oxygen is plentiful the
pyruvate is converted to acetyl coenzyme A (acetyl CoA)
• For aerobic organisms, when oxygen is scarce, and for
some anaerobic organisms, the pyruvate is reduced to
lactate
• For some anaerobic organisms (like yeast), the pyruvate is
fermented to ethanol
Conversion of Pyruvate to Acetyl CoA
• Under aerobic conditions, pyruvate from glycolysis is
decarboxylated to produce acetyl CoA, which enters the
citric acid cycle as well as other metabolic pathways
- the enzyme involved is pyruvate dehydrogenase and the
coenzyme NAD+ is also required
• This pathway provides the most energy from glucose
O
||
CH3—C—COO- + HS—CoA + NAD+

pyruvate
O
||
CH3—C—S—CoA + CO2 + NADH + H+
acetyl CoA
Conversion of Pyruvate to Lactate
• For aerobic organisms under anaerobic conditions, pyruvate is
reduced to lactate, which replenishes NAD+ to continue
glycolysis
• During strenuous exercise, muscle cells quickly use up their
stored oxygen, creating anaerobic conditions
- lactate accumulates, leading to muscle fatigue and soreness
• Anaerobic bacteria can also produce lactate, which is how we
make pickles and yogurt (among other things)
O
lactate
||
dehydrogenase
CH3—C—COO- + NADH + H+

pyruvate
OH
|
CH3—CH—COO- + NAD+
lactate
Conversion of Pyruvate to Ethanol
• Anaerobic microorganisms such as yeast, convert pyruvate to
ethanol by fermentation
- pyruvate is decarboxylated to acetaldehyde, which is
reduced to ethanol
- NAD+ is regenerated to continue glycolysis
• The CO2 produced during fermentation make the bubbles in
beer and champagne, and also makes bread rise
• Alcoholic beverages produced by fermentation can be up to
around 15% ethanol
- above that concentration the yeast die
H+
O
NADH + H+
CO2
NAD+
O
O
OH
O
Pyruvate
pyruvate
decarboxylase
H
Acetaldehyde
alcohol
dehydrogenase
Ethanol
Overview of Pyruvate Pathways
Glycogenesis
• Glycogen is a highly branched glucose polymer used for
carbohydrate storage in animals
• Glycogen stores are used to keep the blood sugar level
steady between meals
• Glycogenesis is the synthesis of glycogen from glucose-6phosphate
- it occurs when high levels of glucose-6-phosphate are
formed in the first reaction of glycolysis
- it does not operate when glycogen stores are full, which
means that additional glucose is converted to body fat
Diagram of Glycogenesis
• Glucose is converted to
glucose-6-phosphate,
using one ATP
• Glucose-6-phosphate is
converted to glucose-1phosphate, which is
activated by UTP,
forming UTP-glucose
• As UTP-glucose attaches
to the end of the
glycogen chain, UDP is
released (and converted
to UTP by ATP)
Formation of Glucose-6-Phosphate
• Glucose is converted to glucose-6-phosphate, using ATP,
in the first step of glycolysis
P O CH2
O
OH
OH
OH
OH
Glucose-6-phosphate
Formation of Glucose-1-Phosphate
• Glucose-6-phosphate is converted
to glucose-1-phosphate
P O CH2
H O CH2
O
O
OH
OH
OH
OH
OH
Glucose-6-phosphate
O P
OH
OH
Glucose-1-phosphate
Formation of UTP-Glucose
• UTP activates glucose-1-phosphate to form
UDP-glucose and pyrophosphate (PPi)
O
CH2OH
H
O
OH
O
O P O
OH
OH
O-
UDP-glucose
O
O
P O CH2
OOH
N
N
O
OH
Glycogen Formation
• The glucose in UDP-glucose adds to glycogen
UDP-Glucose + glycogen  glycogen-glucose + UDP
• The UDP reacts with ATP to regenerate UTP.
UDP + ATP  UTP + ADP
Glycogenolysis
• Glycogenolysis is the breakdown of glycogen to glucose
• The glucose is phosphorylated as it is cleaved from the
glycogen to form glucose-1-phosphate
• Glucose-1-phosphate can be converted to glucose-6phosphate, which can enter glycolysis
• Phosphorylated glucose can’t be absorbed into cells
- in the liver and kidneys, glucose-6-phosphate can be
hydrolized to glucose
• Glycogenolysis is activated by glucogon in the liver and
epinephrine in muscles
- these are produced when blood glucose levels are low
• Glycogenolysis is inhibited by insulin
- insulin is produced when blood glucose levels are high
Overview of Glycogen Synthesis and Breakdown
Gluconeogenesis (Glucose Synthesis)
• Glucose is the
primary energy
source for the brain,
skeletal muscle, and
red blood cells
• Deficiency can
impair the brain
function
• Gluconeogenesis is
the synthesis of
glucose from carbon
atoms of
noncarbohydrates
- required when
glycogen stores are
depleted
Diagram of Gluconeogenesis
• Carbon atoms for
gluconeogenesis come
from lactate, some amino
acids, and glycerol, and
are converted to pyruvate
or other intermediates
• Seven reactions are the
reverse of glycolysis and
use the same enzymes
• 3 glycolysis reactions are
not reversible:
- reaction 1 Hexokinase
- reaction 3
Phosphofructokinase
- reaction 10
Pyruvate kinase
Pyruvate to Phosphoenolpyruvate
• A carbon is added to pyruvate to form oxaloacetate by two
reactions that replace the reverse of reaction 10 of
glycolysis
• Then a carbon is removed, and a phosphate added, to form
phosphoenolpyruvate
Phosphoenolpyruvate to Fructose-1,6-bisphosphate
• Phosphoenolpyruvate is converted to fructose-1,6bisphosphate using the same enzymes in glycolysis
Formation of Glucose
• A loss of a phosphate from fructose-1,6-bisphosphate
forms fructose-6-phosphate and Pi
• A reversible reaction converts fructose-6-phosphate to
glucose-6-phosphate
• The removal of phosphate from glucose-6-phosphate forms
glucose
Cori Cycle
• When anaerobic conditions occur in active muscle,
glycolysis produces lactate
• The lactate moves through the blood stream to the liver,
where it is oxidized back to pyruvate.
• Gluconeogenesis converts pyruvate to glucose, which is
carried back to the muscles
• The Cori cycle is the flow of lactate and glucose between
the muscles and the liver
Pathways for Glucose
Regulation of Glycolisis and Gluconeogenesis
• High glucose levels and insulin promote glycolysis
• Low glucose levels and glucagon promote gluconeogenesis